The presence of water in aircraft fuel has been a topic of concern for the aviation industry for many decades, since any contaminating water may freeze during long flights at high altitudes. The resulting ice particles may block fuel filters or otherwise interfere with the steady flow of fuel to the engines, thereby creating a clear safety issue.
Fuel will absorb water and the quantity will depend on the ambient conditions such as temperature and humidity. The fuel will tend to precipitate any excess dissolved water when it is cooled to the temperatures prevailing at high altitudes. The excess water may then appear in the form of fine water droplets or as ice crystals.
The water that settles out from the fuel will contain small quantities of fuel or components of the fuel. It is known that at low temperatures, unusual complex crystalline structures can be formed with water and small hydrocarbon molecules. These are known as clathrates and they are reported to have a strong tendency to accrete, adhere to pipe walls and to be slow to dissociate.
Heat exchangers are commonly used to heat the fuel to prevent ice from building up in the aluminium tubes that feed fuel from the tanks to the wing mounted engines. Heat moves inward from the heated surface by conduction and convection to warm the fuel. The rate of heat transfer is limited and the heating is not uniform.